It has become somewhat common practice to laminate discrete sheet stock by heating one or two sheets of Mylar coated with polyethylene to a temperature sufficient for heat sealing of the polyethylene and then passing the sheets over the stock and between pressure rolls for final lamination. Various machines have been developed for this type of laminating operation. Such machines require elements for preheating the plastic laminating sheets, which are normally Mylar coated with polyethylene, and means for bringing the sheets into contact with the stock to be laminated and then pulling the sheets and stock through the pressure rolls. One of the most commonly used of these laminating machines incorporates two vertically spaced heating elements over which the Mylar sheets are drawn for heating. The Mylar contacts the heating elements and conducts heat to the polyethylene. These spaced heating elements have curvilinear surfaces facing toward the front end of the machine and are coated generally with a non-stick material, such as Teflon, so that as the sheets of Mylar are pulled over the heating elements to the laminating position they are heated to a sufficiently high temperature for pressure lamination of the polyethylene against the sheet stock to be laminated. Heretofore, this type of machine has been used for laminating discrete sheet stock such as menus, identification cards, place mats, etc. In these applications, the Mylar sheets, after being heated by the heating elements, are pressed against opposite surfaces of the discrete sheet stock and laminated thereto. This machine has proven quite satisfactory for use in this type of sheet laminating.
In the art of laminating, many applications do not lend themselves to use of a standard sheet laminating machine of the type described above. One of these operations is dry mounting wherein a graphic print of some type is adhered to a rigid board stock by an intermediate layer of hot adhesive material, such as polyethylene. In many instances, there is no overlay of Mylar or other plastic material to protect the graphic print, although this is possible in some applications. In a dry mounting operation, the graphic print, such as a photograph, drawing, map, etc., is placed onto a somewhat rigid board stock with an intermediate layer of hot adhesive material, which may be polyethylene. This laminate is heated and pressed together to form a permanent bond between the graphic print and rigid board stock. The above described standard sheet laminating machine is not applicable for this purpose. The only heating occurs at the heating elements which are spaced from the pass or feed line of the workpiece. Consequently, the rigid board stock with an assembled print could not be heated and pressed together. This type of machine has not been used or even suggested for dry mounting although they are inexpensive and available in most laminatng plants.
Heretofore, dry mounting has been performed in platen presses wherein two large, flat heated platens are employed. One platen is generally stationary and the other platen is pivoted into a parallel pressure relationship with the stationary platen. The platens are not completely flat and when the pivoted platen is brought into pressure relationship with the stationary platen, slight variations can exist in the spacing of the platens. In normal operation, a dry mount laminate, including the rigid board, intermediate adhesive layer and graphic print, is placed between the platens and the movable platen is moved into contact with the laminate compressing it between the heated platens. The two platens are then held together for a substantial length of time, which may be in the general range of 45 seconds. Thereafter, the movable platen is shifted from the stationary platen and the dry mount laminate is removed. Variations in the spacing between the platens can cause certain variations, especially when the dry mount laminate is relatively large. In addition, any moisture in the laminate is trapped and causes gas pockets, which prevent adherence over the total surface of the laminate. Even with these obvious disadvantages of using two relatively movable platens, this type of machine is still widely used for dry mounting operations.